Strong magnetic fields of accreting neutron stars Modeling cyclotron resonance scattering features

Schönherr, Gabriele Strong magnetic fields of accreting neutron stars — Modeling cyclotron lines — Accreting neutron stars in X-ray binaries are unique astrophysical laboratories for studying the physics of matter under extreme conditions. Not only does their compact nature lead to an amount of gravity only topped by black hole systems; they can also possess extreme magnetic fields, exceeding the highest magnetic field which has ever been produced on Earth by a million times. These magnetic fields dominate the observed radiation characteristics, the most prominent being pulsed emission. The origin and structure of the magnetic fields, however, is still highly enigmatic. The only direct method currently known for probing the magnetic field of a neutron star is the study of cyclotron resonance scattering features. These features, first discovered in the spectrum of the binary system Hercules X-1, have been observed as absorption lines in the spectra of more than a dozen accreting X-ray pulsars. They form due to resonant scattering processes of high energy photons with quantized electrons in the accreted matter at the neutron star poles. Their line energies are approximately proportional to the surface magnetic field strength of the neutron star. Moreover, the analysis of their shapes is a powerful tool for assessing the fascinating but poorly understood physics of accretion. Today, with the access to data from satellites like BeppoSAX, RXTE, INTEGRAL and Suzaku, the diagnostic potential of cyclotron lines has grown anew: with these instruments the observed cyclotron line features have been energetically resolved in detail. On the other hand, explicit physical models to understand their complex observed shapes are lacking. Phenomenological models are used to obtain their characteristic parameters and to determine the magnetic field strength. The underlying physics, however, are extremeley difficult to assess with such an approach. In the scope of this work, cyclotron resonance scattering features are calculated for typical neutron star spectra using Monte Carlo simulations. The line profiles are inferred under the assumption of physical parameters such as the magnetic field, the accretion geometry, the plasma temperature and optical depth, and the emergent angle of radiation. Based on these simulations, a new interpolation and convolution model is developed for modeling cyclotron lines in X-ray pulsar continua. This model is further implemented as a local model, named cyclomc, into the spectral fitting analysis package XSPEC to allow for a direct comparison with observational data. Results, obtained from fitting cyclotron lines for …